In line with rapid increase in use of fossil fuels, demand for alternative energy or clean energy is increasing. Thus, the field of power generation and electricity storage that use electrochemical reaction is most actively studied.
As a representative example of electrochemical devices using electrochemical energy, secondary batteries are currently used and use thereof is gradually expanding.
Recently, as technology for portable devices, such as portable computers, portable phones, cameras, and the like, continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, research on lithium secondary batteries having high energy density, high operating potential, long cycle lifespan and low self-discharge rate has been underway and such lithium secondary batteries are commercially available and widely used.
In addition, as interest in environmental problems is increasing, research into electric vehicles, hybrid electric vehicles, and the like that can replace vehicles using fossil fuels, such as gasoline vehicles, diesel vehicles, and the like, which are one of the main causes of air pollution, is underway. As a power source of electric vehicles, hybrid electric vehicles, and the like, nickel-metal hydride secondary batteries are mainly used. However, research into lithium secondary batteries having high energy density and high discharge voltage is actively carried out and some of the lithium secondary batteries are commercially available.
A binder is used to provide adhesive strength or binding strength between electrode active materials, and between an electrode active material and an electrode collector. To improve adhesive strength between an electrode collector and an electrode active material, a large amount of a binder is required. However, due to such large amount of binder, capacity and conductivity of an electrode may be reduced.
On the other hand, when a small amount of a binder is used, adhesive strength is not sufficient, and, as such, an electrode may be peeled in processes such as drying, pressing and the like of an electrode and thereby quality defect ratios of electrodes may be increased. In addition, when adhesive strength of an electrode is low, the electrode may be peeled by external shock and thereby battery performance may be deteriorated. In particular, when an electrode is peeled, contact resistance between an electrode material and a collector is increased and, as such, output performance of an electrode may be deteriorated.
Thus, there is an urgent need in the art to study a binder and an electrode material that may have strong adhesive strength so as to prevent separation between electrode active material components or separation between an electrode active material and a current collector when manufacturing an electrode and may have strong physical properties so as to achieve structural stability of an electrode by controlling volume expansion of an electrode active material caused as charging and discharging are repeated and, accordingly, enhance battery performance.
A conventional organic solvent-based binder, i.e., polyvinylidene fluoride (PVdF), does not meet such requirement and thus, recently, a method of using binders prepared by preparing emulsion particles by aqueous polymerization of styrene-butadiene rubber (SBR) and mixing the emulsion particles with a viscosity controlling agent and the like has been proposed and is currently commercially available. These binders are eco-friendly and used in a small amount and thus may increase battery capacity.
However, a viscosity controlling agent such as carboxymethyl cellulose deteriorates battery processability due to high inflexibility while having high viscosity and stability. In addition, the viscosity controlling agent increases electric resistance and, as such, lifespan characteristics are deteriorated.
Therefore, there is an urgent need to develop a binder that enhances lifespan characteristics of a battery, imparts structural stability to an electrode, and has high adhesive strength.